How to customise chat in Roblox (TextChatService)

Example output from a build like this

2 files in this example. Picoo writes this code for your prompt, so file names and sizes vary.

Sample output: ServerScriptService.ChatSetup + StarterPlayerScripts.ChatClient

-- ChatSetup (Script in ServerScriptService). If a tutorial tells you to copy
-- ChatModules out of Chat and edit them, it predates TextChatService and will
-- not work in a new place. Tags, commands and channels are instances now.
local Players = game:GetService("Players")
local TextChatService = game:GetService("TextChatService")

-- Commands are instances. Created by a server Script, the command fires
-- Triggered on the server, which is where the permission check belongs:
-- a command is just a message a client can send.
local cmd = Instance.new("TextChatCommand")
cmd.Name = "HealCommand"
cmd.PrimaryAlias = "/heal"
cmd.Parent = TextChatService
cmd.Triggered:Connect(function(source: TextSource, _text: string)
	local plr = Players:GetPlayerByUserId(source.UserId)
	if not plr or not isAdmin(plr) then return end     -- never trust the alias
	local hum = plr.Character and plr.Character:FindFirstChildOfClass("Humanoid")
	if hum then hum.Health = hum.MaxHealth end
end)

-- ChatClient (LocalScript). OnIncomingMessage runs on the client, so the tag
-- is drawn here, from an attribute the SERVER set: a client cannot award
-- itself [OWNER] in anyone else's chat window.
TextChatService.OnIncomingMessage = function(message: TextChatMessage)
	local props = Instance.new("TextChatMessageProperties")
	local src = message.TextSource
	if not src then return props end

	local plr = Players:GetPlayerByUserId(src.UserId)
	local tag = plr and plr:GetAttribute("ChatTag")     -- server-set attribute
	if tag then
		props.PrefixText = `<font color="#FFD166">[{tag}]</font> {message.PrefixText}`
	end
	return props
end